Non-Aqueous Electrolytic Solution With Mixed Salts

ABSTRACT

The use of at least two electrolyte salts in a lithium secondary battery provides improved battery performance such as long cycle life of high discharge capacity and high capacity retention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/113,966, filed Apr. 25, 2005, the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a field of nonaqueous electrolyticsolutions and a secondary battery using the same. More particularly,this invention pertains to nonaqueous electrolytic solutions thatcomprise (a) one or more solvents and (b) two or more ionic salts. Thepresent invention pertains to secondary batteries comprising suchnonaqueous electrolytic solutions, and particularly to methods of makingnonaqueous electrolytic solutions with at least two salts for use inlithium and lithium ion rechargeable batteries.

2. Description of Related Art

Electrolytic solutions in state-of-the-art lithium ion rechargeablebatteries contain ethylene carbonate (EC) as a co-solvent, and lithiumhexafluorophosphate (LiPF₆) as an electrolytic salt. In the batterysystem, EC must be used in order to form stable solid electrolyteinterface (SEI) that is critical to the cell performance.

LiPF₆ has been used as an electrolytic salt due to its good overallproperties, although it does not have the best individual propertiessuch as ion conductivity, ion mobility, thermal stability, andelectrochemical stability, when compared with other lithium salts suchas lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄),lithium hexafluoroarsenate (LiAsF₆), lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI), etc. However, due to itsthermal instability at relatively high temperatures (see Reaction 1below), LiPF₆-based electrolytic solutions cannot be used attemperatures above 50° C., which limits the cell performance of lithiumion rechargeable batteries containing LiPF₆-based electrolytes in highertemperature applications.LiPF₆→LiF+PF₅  (1)

On the other hand, LiPF₆ is not chemically stable and is easily todecompose by hydrolysis in the presence of residual moisture or acidicimpurities in the lithium salt and solvents (see Reaction 2 below).LiPF₆+H₂O→2HF+LiF+POF₃  (2)

The presence of the strong Lewis acid PF₅ and strong acid HF in theelectrolytic solutions is very harmful to batteries because they reactwith solvent components and electrode active materials and corrodes theSEI, therefore, resulting in the poor long cycle life performance of thebatteries. Hence, there is room for improvement in the selection of anelectrolyte for use in secondary batteries.

SUMMARY OF THE INVENTION

In recent years, lithium bis(oxalato)borate (LiBOB), has been studiedextensively. It has been found that LiBOB-PC based electrolyticsolutions in graphite lithium ion battery systems showed very good cellperformance because LiBOB generates a good SEI on graphite anodes, whichimproves battery performance.

At the same time, due to the very good thermal stability of LiBOB (up to300° C.), batteries with LiBOB-based electrolytic solutions can becycled at high temperatures such as 60° C. or even 70° C. and thebattery performance keeps quite stable after long cycles.

However, the solubility of LiBOB is not high in conventional batterysolvent systems incorporating ethylene carbonate (EC), ethyl methylcarbonate (EMC), dimethyl carbonate (DMC) or diethyl carbonate (DEC), ortheir combinations such as EC/EMC, EC/DMC/EMC, EC/EMC/DEC, etc. Themaximum concentration of LiBOB in these solvent systems is about 0.8˜0.9M (i.e. mole/liter). Simultaneously the ionic conductivities of theseLiBOB-based electrolytic solutions are several mS/cm lower than those ofLiPF₆-based electrolytic solutions. Thus when electrolytic solutionscontaining LiBOB as the sole salt are used in high power lithium ionbatteries, the capacity of the batteries is low and the batteryperformance at high rate charge/discharge is poor.

Therefore it is reasonable to assume that if LiPF₆ and LiBOB are mixedinto conventional battery solvents, the lithium ion batteries using theresulting electrolytic solutions should perform well, especially interms of long cycle life.

The present invention provides a stable nonaqueous electrolytic solutionfor use in lithium and lithium ion secondary batteries, and arechargeable battery using the same. In particular, the presentinvention provides a secondary battery comprising an anode, a cathode,and an electrolytic solution, wherein the electrolytic solutioncomprises a non-aqueous solvent and a solute. The solute comprises afirst lithium salt, and a second lithium salt, different from the first.

The invention further provides a secondary battery comprising an anode,a cathode, and, an electrolytic solution. The electrolytic solutioncomprises a non-aqueous solvent, a first salt comprising LiBOB in aconcentration of over 0.15 M to about 2.0 M and a second salt in aconcentration of about 0.01M to about 2.5 M. The second salt may beselected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiAsF₆,LiTaF₆, LiAlCl₄, Li₂B₁₀Cl₁₀, LiCF₃SO₃, LiE(C_(n)F_(2n+1)SO₂)_(m),wherein m=2 or 3, wherein E=N when m=2, and wherein E=C when m=3, andn=1-10; LiPF_(x)(R_(F))_(6−x), and LiBF_(y)(R_(F))_(4−y), wherein R_(F)represents perfluorinated C₁-C₂₀ alkyl groups or perfluorinated aromaticgroups, x=0-5, and y=0-3, and combinations thereof.

Further, the invention provides a non-aqueous electrolytic solution foruse in a secondary battery comprising two salts, one of which is LiBOBin a concentration of at over 0.15 M.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments describe the preferred modes presentlycontemplated for carrying out the invention and are not intended todescribe all possible modifications and variations consistent with thespirit and purpose of the invention. These and other features andadvantages of the present invention will become more readily apparent tothose skilled in the art upon consideration of the following detaileddescription that described both the preferred and alternativeembodiments of the present invention.

The invention provides a secondary battery comprising an anode, acathode, an electrolytic solution, wherein the electrolytic solutioncomprises a non-aqueous solvent, and a solute comprising at least twosalts. In a preferred embodiment, the first of the salts is a chelatedorthoborate salt or chelated orthophosphate salt. The first salt ispresent in a concentration of over 0.15 M in the electrolytic solution.The invention further provides a non-aqueous electrolytic solution foruse in a secondary battery, wherein the electrolytic solution comprisestwo salts, one of which is LiBOB. The major components, solute salts,solvent, anode and cathode are each described in turn hereinbelow.

Solute. The solutes herein are ionic salts containing at least one metalion. Typically this metal ion is lithium (Li⁺). The salts hereinfunction to transfer charge between the anode and the cathode of abattery. One class of salts includes salts of chelated orthoborates andchelated orthophosphates (collectively, hereinafter, “ortho-salts”). Ina preferred embodiment, the first salt is LiBOB. Other ortho-salts saltsmay be used as well, either instead of or in addition to, LiBOB, forexample, lithium bis(malonato) borate (LiBMB), lithiumbis(difluoromalonato) borate (LiBDFMB), lithium (malonato oxalato)borate (LiMOB), lithium (difluoromalonato oxalato) borate (LiDFMOB),lithium tris(oxalato)phosphate (LiTOP), and lithium tris(difluoromalonato) phosphate (LiTDFMP). Another class of salts usefulherein includes lithium salts that are perhalogenated, or peroxidated,for example, LiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄, Li₂B₁₀Cl₁₀,LiCF₃SO₃, LiE(C_(n)F_(2n+1)SO₂)_(m), wherein m=2 or 3, wherein E=N whenm=2, and wherein E=C when m=3, and n=1-10; LiPF_(x)(R_(F))_(6−x), andLiBF_(y)(R_(F))_(4−y), wherein R_(F) represents perfluorinated C₁-C₂₀alkyl groups or perfluorinated aromatic groups, x=0-5. Any combinationof two or more of the aforementioned salts may also be used.

Broadly, the concentration of salts (first or second salts) in theelectrolytic solution is about 0.01-2.5 M (moles per liter). Preferablythe concentration is 0.05-2.0 M, and more preferably 0.1-1.6M. In allembodiments herein, within the salt ranges given hereinabove, when oneor more chelated orthoborate salts or chelated orthophosphate salts(e.g., LiBOB, LiBMB, etc,) are present the total concentration of suchortho-salts salts should be higher than 0.15 M (e.g, >0.15 M to 2.5M, >0.15 M to 2.0M; >0.15M to 1.5M; >0.15M to 1.0 M). When theortho-salts are present, preferably, they are present in a concentrationof at >0.15 to 2.0 M, more preferably about 0.3-1.6 M, and mostpreferably 0.4-1.2 M. In a most preferred embodiment, LiBOB is presentin a concentration of 0.4-0.8M. Preferably, the first salt is a chelatedorthoborate salt or a chelated orthophosphate salt, or combinationsthereof. Most preferably the first salt is LiBOB.

Solvent. The solvent is a non-aqueous, aprotic, polar organic substancewhich dissolves the solute. Blends of more than one solvent may be used.Generally, solvents may be carbonates, carboxylates, lactones,phosphates, five or six member heterocyclic ring compounds, and organiccompounds having at least one C₁-C₄ group connected through an oxygenatom to a carbon. Lactones may be methylated, ethylated and/orpropylated. Generally, the electrolytic solution comprises at least onesolute dissolved in at least one solvent. Useful solvents herein includeethylene carbonate, propylene carbonate, butylene carbonate, dimethylcarbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate,ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate,tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane,acetonitrile, dimethylformamide, methyl formate, ethyl formate, propylformate, butyl formate, methyl acetate, ethyl acetate, propyl acetate,butyl acetate, methyl propionate, ethyl propionate, propyl propionate,butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate,butyl butyrate, γ-butyrolactone, 2-methyl-γ-butyrolactone,3-methyl-γ-butyrolactone, 4-methyl-γ-butyrolactone, β-propiolactone,δ-valerolactone, trimethyl phosphate, triethyl phosphate,tris(2-chloroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate,tripropyl phosphate, triisopropyl phosphate, tributyl phosphate,trihexyl phosphate, triphenyl phosphate, tritolyl phosphate, andcombinations thereof. Other solvents may be used so long as they arenon-aqueous and aprotic, and are capable of dissolving the solute salts.

In a preferred embodiment, the solvent is selected from the groupconsisting of ethylene carbonate, propylene carbonate, ethyl methylcarbonate, diethyl carbonate and combinations thereof. In anotherembodiment, the solvent comprises about 1-50% by volume (vol %) ethylenecarbonate, and about 1-99 vol % ethyl methyl carbonate. In anotherembodiment, the non-aqueous solvent comprises ethylene carbonate andethyl methyl carbonate in a volume ratio of about 1:4 to about 1:1.

Anode. The anode may comprise carbon or compounds of lithium. The carbonmay be in the form of graphite. Lithium metal anodes may be used.Lithium MMOs such as LiMnO₂ and Li₄Ti₅O₁₂ are also envisioned. Alloys oflithium with transition or other metals (including metalloids) may beused, including LiAl, LiZn, Li₃Bi, Li₃Cd, Li₃Sd, Li₄Si, Li_(4.4)Pb,Li_(4.4)Sn, LiC₆, Li₃FeN₂, Li_(2.6)Co_(0.4)N, Li_(2.6)Cu_(0.4)N, andcombinations thereof. The anode may further comprise an additionalmaterial such as a metal oxide including SnO, SnO₂, GeO, GeO₂, In₂O,In₂O₃, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Ag₂O, AgO, Ag₂O₃, Sb₂O₃, Sb₂O₄, Sb₂O₅,SiO, ZnO, CoO, NiO, FeO, and combinations thereof.

Cathode. The cathode comprises a lithium metal oxide compound. Inparticular, the cathode comprises at least one lithium mixed metal oxide(Li-MMO). Lithium mixed metal oxides contain at least one other metalselected from the group consisting of Mn, Co, Cr, Fe, Ni, V, andcombinations thereof. For example the following lithium MMOs may be usedin the cathode: LiMnO₂, LiMn₂O₄, LiCoO₂, Li₂Cr₂O₇, Li₂CrO₄, LiNiO₂,LiFeO₂, LiNi_(x)Co_(1−x)O₂ (0<x<1), LiFePO₄, LiMn_(0.5)Ni_(0.5)O₂,LiMn_(1/3)Co_(1/3)Ni_(1/3)O₂, wherein Me may be one or more of Al, Mg,Ti, B, Ga, or Si, and LiMc_(0.5)Mn_(1.5)O₄ wherein Mc is a divalentmetal, and mixtures thereof.

Either the anode or the cathode, or both, may further comprise apolymeric binder. In a preferred embodiment, the binder may bepolyvinylidene fluoride, styrene-butadiene rubber, polyamide or melamineresin, and combinations thereof.

The electrolytic solution in the present invention may further compriseone or more additives, such as a sultone (e.g., 1,3-propane sultone, and1,4-butane sultone) to prevent or to reduce gas generation of theelectrolytic solution as the battery is charged and discharged attemperatures higher than ambient temperature, and/or an aromaticcompound (e.g., biphenyl and cyclohexylbenzene) to prevent overcharge oroverdischarge of the battery.

It is envisioned that the salt additives, electrolytic solutions andbatteries discussed herein have a wide range of applications, including,at least, calculators, wrist watches, hearing aids, electronics such ascomputers, cell phones, games etc, and transportation applications suchas battery powered and/or hybrid vehicles.

EXAMPLES

The following compositions represent exemplary embodiments of theinvention. They are presented to explain the invention in more detail,and do not limit the invention.

(1) Preparation of Electrolytic Solutions. Examples 1-4 and ComparativeExamples 1-2. Ethylene carbonate (EC) and ethyl methyl carbonate (EMC)were mixed in a volume ratio of 1:2 to prepare a nonaqueous organicsolvent mixture. Lithium hexafluorophosphate (LiPF₆) and/or lithiumbis(oxalato)borate were added into the solvent mixture, in amountssufficient to give final concentrations shown in Table 1, to give theelectrolytic solutions. All exemplary solutions were formulated atambient temperature (ca. 23° C.). TABLE 1 Electrolytic Solutions:Inventive Examples 1-4 and Comparative Examples A-B. Example/Salt LiPF₆LiBOB Example 1 0.8 M 0.2 M Example 2 0.6 M 0.4 M Example 3 0.4 M 0.6 MExample 4 0.2 M 0.7 M Comparative Example A 1.0 M — Comparative ExampleB — 0.8 M

(2) Preparation of a Cathode. A positive electrode slurry was preparedby dispersing LiCoO₂ (positive electrode active material, 90 wt %),poly(vinylidenefluoride) (PVdF, binder, 5 wt %), and acetylene black(electro-conductive agent, 5 wt %) into 1-methyl-2-pyrrolidone NMP). Theslurry was coated on aluminum foil, dried, and compressed to give acathode. The cathode was die-cut into discs by a punch with a diameterof 12.7 mm.

(3) Preparation of an Anode. Artificial graphite (negative electrodeactive material, 95 wt %) and PVdF (binder, 5 wt %) were mixed into NMPto form a negative active material slurry which was coated on copperfoil, dried, and pressed to give an anode. The anode was die-cut intodiscs by a punch with a diameter of 14.3 mm.

(4) Assembly of a Lithium Ion Secondary Battery. A separate batterycontaining each of the above mentioned electrolytes (Examples 1-4 andComparative Examples A-B) was made by the following procedure. In a drybox under an argon atmosphere, a lithium ion secondary battery wasassembled using a 2032 type coin cell. A cathode was placed on a cathodecan, and a microporous polypropylene film (25 μm thickness and 19.1 mmdiameter) was placed as a separator. It was pressed with a polypropylenegasket, and an anode was placed. A stainless steel spacer and springwere included to adjust the thickness and make good contact. Anelectrolytic solution of each of Examples 1-4 and Comparative ExamplesA-B was added to each of six separate batteries and allowed to absorb.An anode cover was mounted to seal each battery with a crimper, tocomplete the assembly of the coin type lithium ion secondary battery.

(5) Testing of the Batteries. Evaluation of the aforementioned assembledbatteries (e.g., Examples 1-4; Comparative Examples A-B) was carried outin the order (A) initial charging and discharging (confirmation ofcapacity) and (B) cycle life test.

A. Capacity Confirmation. Initial charging and discharging of theaforementioned assembled batteries were performed according to theconstant current/voltage charging and constant current dischargingmethod in a room temperature atmosphere. The battery was first chargedup to 4.2 Volts (V) at a constant current rate of 0.5 mA/cm² (milliampsper square centimeter). After reaching 4.2 V, the battery wascontinually charged at a constant voltage of 4.2 V until the chargingcurrent reached or was less than 0.1 mA. Then the battery was dischargedat a constant current rate of 0.3 mA/cm² until the cut-off voltage 3.0 Vreached. Standard capacity (C) of a nonaqueous electrolyte secondarybattery was 3.4 mAh (milliamp hours).

B. Cycle Life Test. Cycle life test was conducted over 100 cycles atroom temperature by charging the aforementioned initiallycharged/discharged batteries at a constant current rate of C/2 (1.7 mA)to 4.2 V and then charged at a constant voltage of 4.2 V till thecurrent reached or was less than 0.1 mA. After that the battery wasdischarged at a constant current rate of C/2 (1.7 mA) until the cut-offvoltage 3.0 V reached. Discharge capacity retention rate of cycle life(%)=(n^(th) cycle discharge capacity/1^(st) cycle dischargecapacity)×100%. First cycle efficiency is cycle discharge capacity/1stcycle charge capacity×100%. Table 2 displays the results of the lifecycle testing. TABLE 2 Cycle life test results. Discharge capacity1^(st) cycle charge 1^(st) cycle retention Electrolyte capacity (mAh)efficiency 50^(th) cycle 100^(th) cycle Example 1 3.54 95.5% 92.0% 92.9%Example 2 3.58 95.8% 92.4% 93.6% Example 3 3.52 93.8% 94.8% 94.5%Example 4 3.50 94.6% 94.0% 94.3% Comparative 3.43 93.9% 87.3% 85.7%Example A Comparative 3.42 92.4% 91.8% 93.0% Example B

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative example shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general invention concept asdefined by the appended claims and their equivalents.

1-39. (canceled)
 40. A non-aqueous electrolytic solution comprising: a.a non-aqueous solvent, wherein the non-aqueous solvent comprisesethylene carbonate and ethylmethyl carbonate in a volume ratio of about1:4 to about 1:1, b. a first salt comprising lithium bis(oxalato)boratein a concentration of greater than 0.15 M to about 2.0 M and c. a secondsalt in a concentration of about 0.1 M to about 1.6 M, wherein thesecond salt is selected from the group consisting of LiPF₆, LiBF₄,LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄, Li₂B₁₀, LiCF₃SO₃,LiE(C_(n)F_(2n+1)SO₂)_(m), wherein m=2 or 3, wherein E=N when m=2, andwherein E=C when m=3, and n=1-10; LiPF_(x)(R_(F))_(6−x), andLiBF_(y)(R_(F))_(4−y), wherein R_(F) represents perfluorinated C₁-C₂₀alkyl groups or perfluorinated aromatic groups, x=0-5, and y=0-3, andcombinations thereof.
 41. The non-aqueous electrolytic solution of claim40, wherein the second salt is selected from the group consisting ofLiPF₆, LiBF₄, and combinations thereof.
 42. The non-aqueous electrolyticsolution of claim 40, wherein lithium bis(oxalato)borate is present in aconcentration of greater than 0.15 M to about 1.0 M.
 43. The non-aqueouselectrolytic solution of claim 40, wherein lithium bis(oxalato)borate ispresent in a concentration of about 0.4 M to about 0.8 M.
 44. Anon-aqueous electrolytic solution comprising: a. a non-aqueous solvent,b. a first salt comprising lithium bis(oxalato)borate in a concentrationof greater than 0.15 M to about 2.0 M and c. a second salt in aconcentration of about 0.1 M to about 1.6 M, wherein the second salt isselected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiAsF₆,LiTaF₆, LiAlC₄, Li₂B₁₀Cl₁₀, LiCF₃SO₃, LiE(C_(n)F_(2n+1)SO₂)_(m), whereinm=2 or 3, wherein E=N when m=2, and wherein E=C when m=3, and n=1-10;LiPF_(x)(R_(F))_(6−x), and LiBF_(y)(R_(F))_(4−y), wherein R_(F)represents perfluorinated C₁-C₂₀ alkyl groups or perfluorinated aromaticgroups, x=0-5, and y=0-3, and combinations thereof.
 45. The non-aqueouselectrolytic solution of claim 44, wherein the non-aqueous solvent isselected from the group consisting of ethylene carbonate, propylenecarbonate, diethyl carbonate, ethyl methyl carbonate and combinationsthereof.
 46. A non-aqueous electrolytic solution comprising: a. anon-aqueous solvent selected from the group consisting of ethylenecarbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl methylcarbonate, methyl propyl carbonate, ethyl propyl carbonate,tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane,acetonitrile, dimethylformamide, methyl formate, ethyl formate, propylformate, butyl formate, methyl acetate, ethyl acetate, propyl acetate,butyl acetate, methyl propionate, ethyl propionate, propyl propionate,butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate,butyl butyrate, γ-butyrolactone, 2methyl-γ-butyrolactone,3-methyl-γ-butyrolactone, 4-methyl-γ-butyrolactone, β-propiolactone,δ-valerolactone, trimethyl phosphate, triethyl phosphate,tris(2-chloroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate,tripropyl phosphate, triisopropyl phosphate, tributyl phosphate,trihexyl phosphate, triphenyl phosphate, tritolyl phosphate, andcombinations thereof. b. a first salt comprising lithiumbis(oxalato)borate in a concentration of greater than 0.15 M to about2.0 M and c. a second salt in a concentration of about 0.01 M to about2.5 M, wherein the second salt is selected from the group consisting ofLiPF₆, LiBF₄, LiClO₄, LiAsF₆, LiTaF₆, LiAlCl₄, Li₂B₁₀Cl₁₀, LiCF₃SO₃,LiE(C_(n)F_(2n+1)SO₂)_(m), wherein m=2 or 3, wherein E=N when m=2, andwherein E=C when m=3, and n=1-10; LiPF_(x)(R_(F))_(6−x), andLiBF_(y)(R_(F))_(4−y), wherein R_(F) represents perfluorinated C₁-C₂₀alkyl groups or perfluorinated aromatic groups, x=0-5, and y=0-3, andcombinations thereof.